a vast empty plain 空旷的大平原

a vast amount/number/quantity Our dog eats a vast amount of food each day. 我们的狗每天吃大量的食物。

a vast range/array the vast array of computer games available 可买到的多种电脑游戏

in vast numbers/amounts People flocked to the show in vast numbers. 众多的人们成群结队去看演出。

the vast majority of (=almost everyone or everything) I believe the vast majority of people will support us. 绝大多数的

a vast expanse (=a very large area) the vast expanse of the National Park 广阔的地区；大面积

The vastness of the topic overwhelms most readers. 这个题目涉及之广让大多数读者摸不着头脑。